1. Field of Invention
This invention relates generally to apparatus for controlling pressure of a gas in a system that depends on the pressure being produced by bubbling the gas through a liquid, and more particularly to apparatus for effecting bubble continuous positive airway pressure (CPAP).
2. Description of Related Art
Nasal Continuous Positive Airway Pressure (NCPAP) is a well-known technique for providing respiratory support to newborn term and preterm infants with respiratory distress requiring support less aggressive than intubation and mechanical ventilation. The basic mechanism consists of attaching standard infant ventilator tubing to the head of the infant. A pair of nasal prongs, which fit in the infant's nostrils or a face mask which fits over the infant's nose and mouth, are provided between the inflow and outflow limbs of a ventilator tubing. Air flows through the tubing at increased pressure relative to atmospheric pressure by including a partial obstruction at the end of the tubing, beyond the nasal prongs/face mask. The air flowing out of the prongs/face mask is forced into the infant's nasal passages and hence his/her lungs. The amount of respiratory support is a function of the pressure in the ventilator tubing (measured in centimeters of water). It is not described by the amount of flow through the prongs/face mask or by the amount of pressure applied to the lungs, neither of which is known.
There are two typical means of controlling the pressure in the NCPAP tubing. One method is to use a mechanical ventilator which is not set to cycle. Rather it is set to provide continuous pressure. Use of a mechanical ventilator is quite reliable, but increases wear and tear on the ventilator. A dial on the face of the ventilator is adjusted to the desired pressure which is read on a manometer and/or a digital readout.
The second method is called “bubble CPAP” and one typical conventional prior art set-up for effecting it is shown in FIG. 1. Thus, as can be seen in that figure, a supply of oxygen from a wall outlet (not shown) is provided through a section suitable conventional tubing 12 to one input of a mixing chamber 14. A supply of air from another wall outlet (not shown) is provided through another section of suitable conventional tubing 16 to another input of the mixing chamber 14. The oxygen and air are mixed within the chamber 14 and flow out of the chamber into a conventional humidifier 18, where the gas mixture is humidified to the desired humidity. The upstream end of one section or limb 20A of a conventional ventilator tube 20 is connected to the outlet of the humidifier 18. The opposite end of that ventilator tube section is connected to the upstream end of either a pair of conventional nasal prongs 22 or to the upstream end of a conventional face mask 24 (depending on which device is to be used to effect the delivery of the air into the nasal passages of the infant). The downstream end of the pair of prongs 22 or face mask 24 is connected to the upstream end of another section or limb 20B of the ventilator tube 20. The downstream free end 20C of the ventilator tube section 20 is submersed in a liquid 26, e.g., 1.25 ml glacial acetic acid added to 500 ml water (0.25% acetic acid), in a bottle 28 or other hollow vessel. Accordingly, the humidified air flows from the humidifier through the ventilator tube section 20A to either the nasal prongs or face mask (as the case may be) and from there into the lungs of the infant. Uninhaled air flows past the nasal prongs or face mask into the downstream ventilator tube section 20B and out its open end 20C, where the exiting gas forms bubbles 30 which rise to the surface of the liquid 26 and dissipate to the ambient atmosphere.
The depth at which the downstream end 20C of the ventilator tube section 20 is located below the surface of the liquid 26 in the bottle determines the pressure in the ventilator tube 20 at the nasal prongs or face mark (i.e., the amount of respiratory support provided to the infant). This pressure is typically measured by means of a tape strip 32 secured to the bottle. In particular, the tape strip has a scale in the form centimeter markings 34 and associated numeric indicia printed thereon so that the position of the downstream free end 20C can be read off of the scale.
The pressure range in the ventilator tubing 20 is typically varied from 4 to 8 centimeters of water, in 1 centimeter increments, by adjusting the height at which the free end 20C of the ventilator tube 20 is located below the surface of the liquid 26. Once the desired distance below the surface of the liquid has been established the tube section 20B is secured in place by some auxiliary means, e.g., wedged with a syringe (not shown) or some other means to hold its free end 20C at the desired depth in the liquid. A pressure manometer 36 may be used along with or in place of the tape strip to determine the pressure in the ventilator tubing.
As will be appreciated by those skilled in the art, while it is possible using a conventional bubble CPAP set-up like that shown in FIG. 1 to place an infant on 5 centimeters of CPAP or 6 centimeters of CPAP, intermediate and finer adjustments, e.g., 5.5 centimeters of CPAP, are not practical. Thus, while the system is an analog one, changes are typically made in whole centimeter increments. Moreover, the use of bubble CPAP requires more diligence for reliability. Specifically, no reliable method for insuring the depth of insertion of the ventilator tube in the liquid is available. Hence the tube may slip in or out inadvertently which changes the pressure in the ventilator tubing to the infant.
Accordingly a need exists for a bubble CPAP system which overcomes the disadvantages of the prior art.